Alas, it is so easy in Java to do operator overloading in Java is missing. You need to write a parser and an interpreter.
But! The easiest option is to use scripts or classes in groovy to Java with very good friends. The interpreter and the parser is already written for you. Download groovy, connect jar to your project, and will Westie.
assert Eval.me('33*3') == 99
assert Eval.me('"foo".toUpperCase()') == 'FOO'